Generating a RIN refers to the process of creating a new RIN to represent a particular type and volume of renewable fuel. See regulation Section 80.1126(d). Assignment occurs when the producer or importer of the renewable fuel transfers a RIN to another party along with a volume of renewable fuel. See regulation Section 80.1126(e).
Note that the regulations do not specify the point when generation of RINs must occur. Under regulation Section 80.1126(e)(2), it is only at the point when a volume of renewable fuel leaves the production or importation facility where it originated that RINs must have been generated for, assigned to, and transferred with that volume. Since the EPA does not specifically define the point of production or importation, a producer can generate RINs as the renewable is being physically produced, as it sits in a tank awaiting transfer to another party, or even while the renewable fuel is being transferred to another party.
Question and Answer was originally posted at: Questions and Answers on the Renewable Fuel Standard Program (PDF) (55 pp, 221 K, EPA420-F-07-041a, August 2007, About PDF)